In these days of heightened, ecological awareness, the rapid detection and cleanup of hydrocarbon spills has become an economic and public relations necessity. One of the primary variables to be determined at any hydrocarbon product spill site is the amount of the hydrocarbon that has spilled into the environment. The quantity of the hydrocarbon spilled is the main factor in determining whether the spill site needs remediation and selecting an appropriate remedial response.
However, the determination of this variable is usually extremely difficult. Some of the hydrocarbon spill is lost due to volatilization and biodegradation. A significant portion of the spill remains absorbed and immobile on soils through which the spill percolates. In a great number of cases, the spill volume is large enough to infiltrate through the earth surface to aquifiers, permeable subterranean formations where the soil is saturated with water. At these locations, the "free" or mobile hydrocarbon begins to pool. As this free hydrocarbon begins to diffuse, some of it is dissolved in the adjacent ground water and removed from the site of the spill via the flow of the ground water. All of these quantities are difficult to measure and become more difficult as time elapses.
A traditional method of estimating the free volume of the hydrocarbon spill has involved the use of monitoring wells. The monitoring wells are placed at various locations around the spill and the thickness of the hydrocarbon layers are measured at these locations using various methods. These thickness values at various points in the aquifer are used sometimes without modification, to draw a contour map of product thickness. The volume of the free hydrocarbon in the aquifer is then determined by using a planimeter on the contour map. The free hydrocarbon volume was taken to be the aquifer product thickness value best determined times the porosity times an assumed value for product saturation. This method grossly overestimated the recoverable hydrocarbon volume at many sites which in turn resulted in unnecessary expense because of the over-design of recovery facilities.